(1) Field of the Invention
The present invention relates to a method for measuring the distance between the vehicle hull and the vapor-liquid cavity formed about an underwater supercavitational vehicle. More specifically, the present invention relates to a method of mounting optical emitters and optical receivers to an underwater vehicle to determine the distance to a vapor-liquid cavity.
(2) Description of the Prior Art
Underwater supercavitational vehicles travel at high speed in the water creating a gaseous cavity around their hulls. The cavity helps support the vehicle's high speed by reducing vehicle drag. The gaseous cavity can either be “ventilated” wherein gas is introduced or self-cavitating wherein low pressure created by hydrodynamic flow induces cavitation of the surrounding fluid. Stable guidance of such a vehicle in the water is critically dependent upon maintenance of this cavity. As the vehicle travels in the water, the cavity shape continually changes, particularly when the vehicle turns. Knowledge of the location of the cavity boundary is useful for maintaining vehicle stability in the water.
Information on the cavity boundary location and contour are also useful for initial hydrodynamic studies in propulsion and cavity ventilation during test exercises and vehicle design.
One technique for measuring the distance between the laser sensor device such as the vehicle hull and a generic (optically reflective) target is through a geometrical triangulation method, as illustrated with reference to FIG. 1. With this technique, a laser beam 11 is transmitted from a transmitter 15 towards a flat target measurement surface 13 at a known angle α. The laser beam 11 is then diffusely reflected from the measurement surface 13 and is recorded by a detector 17 angled appropriately to capture the diffuse reflections from the target surface 13. Since the locations of the transmitter 15 and receiver 17 are fixed, the distance between the transmitter 15 and receiver 17, x, is known as is the launch angle, α, and receive angle, β. The altitude of the triangle, h, can then be calculated to obtain the desired distance information.
However, when the surface is not diffusely reflective, as is the case with the specularly reflecting vapor-liquid interface, the alignment of the sensor becomes difficult. Since the vapor-liquid interface of the underwater supercavitational vehicle will be in continual dynamic motion, the measurement surface is not continuously flat, but rather randomly fluctuates, skewing the reflectance angle. The geometrical based sensor also demands a certain standoff distance in excess of 2 cm, the minimum laser to target surface distance that can be measured. Thus, the triangulation method is not directly suitable for dependably measuring the laser to surface distance. Ranging has also been attempted using heterodyne methods in a backscatter-modulated laser diode setup. The frequency difference between the frequency modulated laser light transmitted and reflected from the target is used to calculate the corresponding laser to target distance variations, about a nominal distance. This nominal distance of the cavity wall location may not be available a priori making sensor initialization difficult. Also, such a phase change in the laser light may come from the temperature gradient within the cavity due to ventilation and rocket motor exhaust gases. The change in phase of the laser light by means other than the cavity wall displacement will effectively reduce the received signal-to-noise ratio and limit sensor capabilities. Thus, it is believed that phase dependent ranging methods are not appropriate for this environment. It is postulated that the proposed method for measuring the cavity wall will not depend upon the frequency modulation of the laser beam or in its phase as the laser beam propagates in the area between the laser and the target.
What is therefore needed is a method for measuring and an apparatus for so measuring the extent of the vapor-liquid cavity formed about an underwater supercavitational vehicle that is not adversely affected by the non-diffuse and variable surface angle reflectivity of the vapor-liquid boundary.